Improve Structure tests

[janosh]

Most notable user-facing change: add in_place: bool = True keyword to Structure.make_supercell() and return the supercell.

a70f4c4 add Structure/Molecule.sites setter, move @property to SiteCollection
7f510ca don't use implementation detail (Structure|Molecule)._sites
c5052a2 rename StructureTest.structure attr to struct
28a895f add IStructureTest.test_sites_setter
185d2cb fix mypy
94f4413 test translate/rotate_sites IndexError if "list index out of range"
2ffd3bb tweak doc strings
f482ba0 test inverse translate_sites operations leave structure unchanged
7197592 Structure.make_supercell() add in_place: bool = True keyword and return supercell structure
638c095 test make_supercell in_place=True and False